A known optical system includes a first optical element having negative refractive power and a second optical element having positive refractive power. The first optical element causes forward light to be transmitted therethrough and causes lateral light to be reflected at a reflection surface located at the image side. The second optical element combines the two light paths from the front and lateral directions via the first optical element so as to form images thereof on a single image acquisition element. The optical system thus has a super-wide field angle so that both forward and lateral images can be observed at the same time (for example, see Patent Literatures 1 and 2).
In the optical system in Patent Literature 1, lateral light is refracted after being incident on a cylindrical surface of the first optical element, is sequentially reflected once at the reflection surface located at the image side and once at a reflection surface located at the object side opposite from the image side, and is subsequently transmitted through a transmission surface provided near the central axis at the image side so as to be incident on a positive optical element.
In Patent Literature 2, lateral light reflected only once at the reflection surface at the image side of the first optical element and forward light refracted after coming from the object side and transmitted through the transmission surface at the image side are incident on the positive optical element.